Switching units for generating an output voltage as a function of a digital data value are used in regulators for instance. In such cases a digital data value of an actuating variable is calculated at regular intervals by means of an algorithm for regulating a process. This calculated data value is then converted into an output voltage in order therewith to control an actuating element. The actuating element thus controlled reduces a control error in the process. The switching unit for generating the output voltage as a function of the digital data value is also referred to as an analog output stage of a (digital) regulator.
The relationship between the digital data value, which is assigned to a switching unit on the one hand and the output voltage actually generated herefor on the other hand is referred to as the characteristic curve of the switching unit. This characteristic curve generally deviates from a theoretical and/or desired characteristic curve. This arises as a result of tolerances which have to be accepted in components of the switching unit and during manufacture of the switching unit.
The deviation of the characteristic curve of a switching unit from the theoretical characteristic curve has the unwanted result, in the case of regulators for instance, of a new control error being generated in the process when controlling the actuating element by means of the switching unit.
To this end, it is known from the prior art for today's regulators not to assign the digital data value directly to the switching unit but instead firstly to calculate a modified data value by means of a mapping function. With this modified data value, the deviation of the characteristic curve of the analog output stage from a desired characteristic curve is then taken into account. The disadvantage of this method is that additional calculating steps are needed to calculate the modified data value. This requires additional calculating capacity and additional calculation time. As a result, the maximum possible rate of change of the output voltage is limited as a function of regularly generated, digital data values and thus the possibility of regulating rapidly changing processes.
The document U.S. Pat. No. 4,342,983 describes a dynamically calibratable digital-to-analog converter, which comprises a conventional, digitally controllable current source and an additional current source. The conventional, digitally controllable current source has a fixed predetermined characteristic curve, which has non-uniform quantization stages. Overlaying an output current of the digitally controllable current source and a current of the additional current source produces, in a summing point, a characteristic curve of the dynamically calibratable digital-to-analog converter with a uniform level of quantization stages.
The document US 2005/0001747 A1 describes a system in which an analog output voltage is generated from a digital input value by means of a digital-to-analog converter. A correction voltage is generated with the aid of a calibration circuit, said correction voltage being overlayed via a summing unit on the output voltage of the digital-to-analog converter. The behavior of the digital-to-analog converter itself is not calibratable here. The correction voltage is generated within the calibration circuit by means of a second digital-to-analog-converter.
The article “Electronic interfaces” by Falconi et al. (Falconi et al.: “Electronic interfaces”, ELSEVIER SEQUOIA S.A., LOUSANNE, CH, Vol. 121, No. 1, Jan. 23, 2007, pages 295-329) describes a circuit for measuring a resistance of a sensor, with the circuit enabling the resistance to be digitally measured across a large range of possible resistance values. To enable an accurate measurement of the resistance value across the large range, a supply current from the sensor can be varied by adjusting a series resistance of a digital-to-analog converter. The series resistance is adjusted by selecting one of a number of series resistances. Similarly, a supply voltage of the sensor can be varied across a further digital-to-analog converter. The digital-to-analog converter and the series resistances cannot be changed in terms of their characteristic curves and do not need to have been calibrated prior to commissioning the circuit.